It takes 30 minutes before I start eating my spa treatment. As a vigorously healthy Swede massages my back, I pluck a large seaweed leaf from the foaming briny and chew like a fat, floating cow.

Seaweed may feature frequently in modern spa therapies, but it's nearly always a concentrated extract. That would be sacrilege at Ljungskile Tångbadhus, 40 minutes north of Gothenburg. One of two traditional bathhouses left on Sweden's serenely beautiful west coast, it turns a daily harvest of fresh seaweed into a luscious bathing brew and highly effective massage sponge.

After an initial 30-minute coastal yomp supplying lungfuls of fresh Bohuslän air, the therapy takes an unusual turn. Either take your healthy dip in the 1923 wooden bathhouse or pay to join the daily harvest. Owner Gunnel Ideberg drives me through dense pine forest to the beautiful swimming spot at Saxenhov, where she uses a knife and trident to gather gloriously scented brunalg seaweed.

"I pull on the waders, walk into the sea and I'm blown away," she says. "I breathe, think and feel differently. It's therapy for people who've hit the wall."

Thirty minutes later and the ocean's bounty is washed and added to wooden tubs of briny, pumped from the sea and warmed to 38C. Its gelébildande polysachander mucus immediately seeps out, generating a brownish, frothy tea.

Being English and pasty, with no genetic link to Freddie Ljunberg, I'm not a natural nudist. But the seaweed forms a leafy G-string, or perhaps sea-string, and it's easy to relax as Gunnel begins to massage. It feels sensational. Some of the weed has tiny white parasitic nodules that act like fine sandpaper, stimulating circulation. At the same time, the brunalg supplies a small pharmacy of vitamins and minerals, which the warm seawater helps pass through the skin's membrane. Gunnel calls it "treatment for the soul" that also balances PH levels and soothes muscles and joints.

Historic Ljungskile Tångbadhus may not be the smoothest surroundings with a tired sauna, 70s curtains and plain walls, but it's unquestionably the most relaxing, enjoyable and effective spa treatment I've ever had.